PL4000 LED stays ON after power switched to OFF

[mslupczynski]

I have a restored PL4000 that after a few weeks of running perfectly with no issues at all, now will not (or appears) not to shut down completely.

Yesterday, I turned it on and the connected 400 amp and 5000 receiver came on as expected but there was a louder than normal hum coming from the setup. Usually I can hear the hum of the 400 transformer a bit, but it's not too bad. This time the hum was significantly louder, so I shut the system down. The 400 and 5000 shut down as expected and the PL4000 internal relay disengaged, or at least it makes the disengagement sound like normal. But the PL4000 pilot light remained on until I unplugged the PL4000. Looking at the schematic I can see that the LED is clearly in the power circuit but I'm left wondering why the LED is on and IS the unit actually on, or just appears to be on due to the LED being ON. Possibly a component failure that leaves a sufficient voltage to the LED but the unit and signal path and non-operational? The AC outlets on the preamp appear to be working normally.

I suspect perhaps this behavior has been seen before by someone out here and thought I'd ask. If you have seen this behavior, or have a recommendation for further trouble shooting I would appreciate any assistance.

 

[laatsch55]

I would check C38, C39 for leakage..

 

[mslupczynski]

I would check C38, C39 for leakage..

That makes sense. So, to me C38 and 39 appear like snubber caps on the relay? Do you suspect that the PL4000 in NOT actually still ON after switch OFF, but only appears ON due to the caps mentioned possibly being defective?

 

[WOPL Sniffer]

Maybe the power switch contacts are welded.

 

[J!m]

Dirty contacts, no snubbers, high draw… weld.

 

[Ronnie_B]

I have a 4000 that does the same. The power switch breaks the transformer secondary, and looks like some caps may provide DC for to forward bias the transistor for a little while. The relay only controls the switched power outlets...

 

[Ronnie_B]

If I turn the 4000 off with the front control panel switch, the LED wll stay lit for about 1 minute, and music continues for about 20 seconds as the DC bleeds down.

Killing the power source is immediate, so I now use a power strip switch to totally kill power to the amp/ preamp. May help the amp/preamp switches live another 50 years or so as well.

 

[wattsabundant]

The contact only provides base drive to the pass transistor in the power supply. The design allows the use of a low power switch. The condition stated above suggests Q1 has shorted.

 

[Ronnie_B]

The contact only provides base drive to the pass transistor in the power supply. The design allows the use of a low power switch. The condition stated above suggests Q1 has shorted.

If his 4000 stays on continuously with the power switch off, then yes Q1 is probably shorted. In my case Q1 turns off after a cap (C6?) discharges which takes about 1 minute after the front control panel switch is turned off. The LED power indicator will stay on for about 1 minute then fade off.

 

[Ronnie_B]

If his 4000 stays on continuously with the power switch off, then yes Q1 is probably shorted. In my case Q1 turns off after a cap (C6?) discharges which takes about 1 minute after the front control panel switch is turned off. The LED power indicator will stay on for about 1 minute then fade off.

I just timed it. After turning power off at the front control panel the LED stays bright for about 20 seconds, starts dimming, and goes dark after about 30 seconds total. I previously overhauled this preamp by replacing transistors, caps, diodes, etc. and circuit behavior was same (as expected).